1. Field of the Invention
The present invention relates to device and method for detecting a synchronizing signal from a digital TV signal.
2. Description of the Related Art
In general, the digital television broadcasting system adopted by the ATSC(Advanced Television Systems Committee) of the U.S. employs terrestrial VSB (Vestigial Side Band) transmission system, in which video data and the synchronizing signal are transmitted separately. And, an exact position of the synchronizing signal should be detected from the video data at a receiver side. Because the synchronizing signal distinguishes between preset intervals of the video data, detection of the exact position of the synchronizing signal is very important. In other words, because the synchronizing signal is a reference timing for processing signals by various parts in the receiver, the exact detection of the synchronizing signal at a reception terminal of the video data has very important. In general, for this synchronizing signal detection at the receiver side, a synchronizing signal pattern is inserted in the video data.
As shown in FIG. 1 which illustrates a general VSB transmission data, the VSB transmission system uses a data stream (or a bit stream) having a particular periodic synchronizing signal pattern inserted thereto for easy detection of the synchronizing signal therefrom in a digital television receiver. The data stream has a succession of data segments of 832 symbols each with a synchronizing signal interval of 4 symbols and a data interval of 828 symbols. The synchronizing signal interval has two levels of `-1` and `+1`, and the data interval has 8 levels.
A background art device for detecting a synchronizing signal from a VSB transmission signal, which is a digital television broadcasting signal, will be explained with reference to the attached drawings. FIG. 1 illustrates a waveform showing a general configuration of a data, FIG. 2 illustrates a block diagram showing a background art device for detecting a synchronizing signal from a VSB transmission signal, and FIGS. 3a to 3c illustrates a waveform showing an example of synchronizing signal detection by means of the device in FIG. 2.
Referring to FIG. 2, the background art device for detecting a synchronizing signal from a VSB transmission signal is provided with a correlator 11 for subjecting the VSB transmission signal received as a datastream and a synchronizing signal pattern preset therein to a correlation operation, a storage 12 for storing a value from the correlator 11, an adder 13 for adding a stored value in the storage 12 and the value from the correlator 11, a limiter 14 for limiting a value from the adder 13 not to exceed a certain plus level, and a confidence counter 15 for detecting a signal exceeding the certain plus level as a synchronizing signal and removing a synchronizing signal misdetected, momentarily.
The operation of the background art device for detecting a synchronizing signal having the aforementioned system will be explained.
Upon reception of a bitstream of VSB transmission signal, the correlator 11 shifts, and subjects the VSB transmission signal by one symbol unit to a correlation operation with a synchronizing signal pattern preset therein, and provides results of the operation in sequence. In the correlation operation in which the synchronizing signal pattern of "+1, -1, -1, +1" is multiplied to values of the shifted data opposite to one another respectively and resultants of the multiplifications are added together, when the present shifted data is the synchronizing signal pattern, the synchronizing signal preset therein and the shifted synchronizing signal pattern are subjected to a correlation operation, to produce 4 which is the maximum. That is, if the synchronizing signal pattern is "+1, -1, -1, +1", because the shifted synchronizing pattern will also be "+1, -1, -1, +1" identical to the synchronizing signal pattern, the correlation operation becomes to produce the maximum value of 4. In the data shifting and subjecting to a correlation operation, a result of correlation operation of an interval in which the preset synchronizing signal pattern meets the synchronizing signal interval in a bitstream produces a signal of which level is higher than a result of correlation operation of an interval in which a data interval meets the preset synchronizing signal pattern. Then, as shown in FIG. 1, the storage 12 is provided with 832 memories considering that the synchronizing signal is repeated at every 832 symbols. Values from the correlator 11 are stored in the respective memories in the storage 12 in sequence. The adder 13 adds a stored value in the storage 12 and a value from the correlator 11, to update the stored value in the storage 12. When the stored value in the storage 12 is kept updated by the addition of the values from the correlator 11 of the adder 13, the stored value falling on the synchronizing signal interval is kept increasing while the same falling on intervals other than the synchronizing signal interval approaches to `0` because data intervals other than the synchronizing interval have random patterns, that is, because positive (+0 values and negative (-) values are added in the adder 13, updating the stored value of the storage 12. However, upon repetition of this operation, peak values shown in FIG. 3a increase continuously to diverge at the end, causing malfunction of synchronizing signal detecting circuit. Accordingly, the limiter 14 is provided for limiting a sum of the value from the correlator 11 and the value from the storage 12 produced from the adder 13 to a certain plus level. Then, the confidence counter 15 detects a position having a value over a certain level from the values of the limiter 14 as a position of the synchronizing signal and conducts confidence counting for reducing mistakes in the detection caused by a momentary great noise, to obtain exact positions of the synchronizing signals as shown in FIG. 3a, finally. In the meantime, if an great external noise is occurred at a position of the bitstream a distance far from a position of the synchronizing signal due to ghost and the like during data reception, in principle, the noise shown in FIG. 3b has a level lower than a detected synchronizing signal, the noise does not affect the synchronizing signal detection, significantly.
However, as shown in FIG. 3c, the synchronizing signal is limited to a certain level by the limiter 14 in the background art, distinguishing the synchronizing signal from the noise signal is not practicable. Therefore, because of the limiting operation for preventing divergence of a provided value, the background art device for detecting a synchronizing signal from a VSB transmission signal has a problem in that a wrong synchronizing signal may be detected due to the limiting level at an occurrence of noise over a certain level.